In power supply systems there is often a need to convert a multi-phase voltage from the mains into a unidirectional current for supplying different kinds of electronic equipment, such as telecommunication equipment.
In such multi-phase AC/DC converters there is usually provided a number of single phase converters or single phase modules (SPM) in order to output a rectified current. However in such AC/DC converters it is sometimes not possible to access the neutral wire of the mains, which can be required for control purposes of the AC/DC converter. Thus, in order to make a multi-phase AC/DC converter operate safely, having control of the input voltage of each SPM and at the same time generate a correct output signal, the control mechanism of the AC/DC converter requires access to a neutral point. Thus, in order to make a multi-phase AC/DCconverter having SPM:s connected in a star connection operate safely, the common point of the SPM:s have to be connected to a stable neutral point.
In the case when there is no access to a neutral point from the neutral wire of the mains, an artificial neutral point (ANP) can be created by means of using three small transformers having their secondary sides connected in a delta configuration.
Such a circuit is described in the international patent application PCT/SE99/00744, which is incorporated herein by reference.
A problem when using the circuit as described in the international patent application PCT/SE99/00744 is that if there is an asymmetry in the supply voltage from the mains, i.e. that the sum of the phase voltages in the mains does not become zero, there will be a current in the circuit forming the artificial neutral point, which in turn will increase the power loss in the artificial neutral point of the converter. Such a power loss is of course a disadvantage. A current in the artificial neutral point may also origin from different input power to different SPM:s, the SPMs may have different efficiency or there may be an asymmetry in the supply voltage.
A circuit can be provided in order to minimize the power loss. Such a circuit is described in the international patent application PCT/SE99/00743, which also is incorporated herein by reference. Thus, in order to reduce the current in the artificial point circuit, a control circuit which measures the current in the delta configured loop of the artificial neutral point (ANP) is provided.
In FIG. 1 a diagram illustrating a 3-phase AC/DC converter as described in the international patent applications PCT/SE99/00743 and PCT/SE99/00744 is shown. Thus, the converter comprises a unit for deriving an artificial neutral point (ANP) 109 connected to the mains R,S,T, which supplies a signal indicative of asymmetries in the mains R,S,T to a control circuit 110. The control circuit controls the single phase converters or single phase modules (SPM) 103, 105 and 107 according to an algorithm designed to eliminate or minimize any current in the artificial neutral point.
However, in order to operate properly the control circuit 110 requires a signal indicative of the current on the secondary side of the ANP. A straight forward solution is to provide a current sensing unit directly in the neutral wire. Another possible solution is to detect the current in the loop on the secondary side in the ANP. Such solutions have however turned out to be costly and a simpler and cheaper method of providing a signal indicative of the current in the loop of the secondary side is therefore desired.